This invention generally relates to the field of measurement of the fill level of the contents of containers at least part of which is transparent, and more particular to optical means by which the height of fill can be measured in such containers, for example glass or plastic bottles, whether of clear or colored glass or other material. The term "transparent" is used herein to mean capable of transmitting electromagnetic radiation at an adopted working wavelength band, not necessarily within just the visible region. For example, certain embodiments of the invention use infra-red radiation, and can thus be used with glasses which are transparent in the infra-red though not in the visible.
Although the invention may be used for other purposes, for example measuring the height-of-fill (HOF) of pulverulent (e.g., powder) materials in glass ampoules, it is primarily intended to address the problem of measuring the HOF of the fluid contents of transparent beverage containers, such as soda or beer bottles, to determine if they meet specifications for their volumetric content, which directly correlates to HOF for a known bottle geometry. While a bottle could possibly be stationary it is typically moving at appreciable speed while conveyed along a production line, so it is measured while moving, but at sampling rates that are sufficiently fast so that the bottle appears "stopped" to the measuring optical system. Preferred embodiments of the invention have been tailored to meet the needs of bottled beers in clear, green and brown glass containers. As will be seen, the system preferably utilizes a line scan CCD imager to provide high-speed image capture.
Height of Fill is traditionally measured using an x-ray beam that is set to 3 mm below the calibrated liquid level. When the beam signal changes, the fill is within limits, and produces a yes/no only answer to a Host control system, usually in the form of a computer of other suitable microprocessor based system.
Optical systems for measuring HOF are also known. For example, U.S. Pat. No. 3,232,429 (Norwich) describes a fill level detector for use in checking the HOF of bottles of foamable liquid passing along a filling line. The presence of a bottle at a measuring station is detected by a mechanical switch, and two beams from light sources are passed through the upper end of the bottle on to two photodetectors. Depending upon the output from the photodetectors, the bottle is classified as acceptably filled, underfilled, or empty, and the latter two categories of bottle are removed from the filling line by a mechanical rejection device. Since this apparatus uses only two spaced photodetectors, only one of which detects the presence of liquid contents lying beneath the foam in the bottle, it gives only a yes/no answer as to whether the bottle is sufficiently filled and cannot measure the actual HOF. Furthermore, since the HOF detection is made from fixed apparatus lying alongside the filling line, the process cannot compensate for vertical variations in the position of the bottle such as tend to occur when bottles are being transported by a flexible conveyor.
U.S. Pat. No. 5,414,778 (Schwartz et al.) describes an apparatus for measuring HOF of a carbonated or similar fluid which is in a "dynamic state" immediately after filling of a bottle. The somewhat elaborate process described in this patent requires line-by-line analysis of a two dimensional image of a part of the bottle, above and below the fill line produced by a video camera; this analysis serves to identity not only the HOF but also the number and size of bubbles within the liquid. To carry out such an image analysis on bottles on a commercial filling line, which may be filling 1200 bottles per minute, requires a high powered computer, and an expensive apparatus. Furthermore, the process described in this patent does not attempt to allow for the presence of foam above the liquid, despite the fact that such foam is inevitably present when many liquids, for example beer, are filled on high speed filling lines.
The present invention provides a process and apparatus for determining the HOF of a container; this process does not require the elaborate analysis and computing power required by the Schwartz et al. process, but can measure the HOF and allow for the presence of foam above the liquid. Preferred embodiments of the present invention can compensate for variations in the vertical height of the container, such as those which occur when the container is being transported by a flexible conveyor.